This invention relates to an improvement to reactors intended for hydroformylation. The hydroformylation reaction, also known as Oxo synthesis, consists in reacting a synthesis gas made up of a mixture of carbon monoxide and hydrogen with an olefin containing "n" carbon atoms so as to obtain a mixture of aldehydes and primary alcohols containing n+1 carbon atoms. The reaction is generally catalysed with carbonyls of transition metals, more particularly cobalt hydrocarbonyl or dicobalt octocarbonyl. This type of reaction is described in U.S. Pat. No. 3,188,351.
The reactor generally used for this type of continuous reaction is of the "loop" type in which the liquid phase is recycled according to the gas pump principle. This reactor is made up of at least two parallel vertical tubes connected to each other at their upper and lower ends. At least one of the vertical tubes acts as the ascending branch and is continuously supplied at its base with the synthesis gas and liquid phase while at least one vertical tube acting as the descending branch allows only the liquid phase to circulate, and all the synthesis gas and the excess liquid phase are evacuated at the upper connection. To achieve this, the reactor comprises, at its top, a broad bell-mouth which makes it easier to separate all the synthesis gases from the liquid phase which is to be recycled. The difference between the specific gravities of the gas/liquid phase mixture on the one hand and the liquid phase alone on the other hand results in a difference in hydrostatic pressure between the ascending branch and the descending branch, thus leading to circulation of the liquid phase in the reactor.
These types of reactors are entirely satisfactory with regard to the circulation of the liquid phase, but have serious drawbacks with regard to the hydroformylation catalyst. The transition metal carbonyls used as catalysts are known to be unstable products which tend to decompose thereby producing a metal compound and giving off carbon monoxide. This is why Oxo synthesis is carried out under pressure as the carbonyls are all the more stable as the partial pressure of carbon monoxide is higher. In the reactors described above, the liquid phase, which also contains catalyst, circulating in the descending branch is totally free of any free synthesis gas and is subject to conditions in which the carbonyls are very unstable thus leading to the decomposition and depositing of metal compounds on the walls of the descending branch. This eventually leads to plugging this branch. Moreover, the productivity of the reactors is reduced owing to the lack of reactive synthesis gases in the descending branch.
It has been found that, by modifying the geometry of the upper connection of the vertical tubes, it is possible not only to eliminate the above-mentioned disadvantages relating to the catalyst, but also to increase the productivity of the reactors considerably. This improvement, the purpose of which is to allow synthesis gas to be entrained into the descending branch where, according to the prior art, only the gas-free liquid phase circulates, is characterised in that the upper connection of the vertical tubes is a tubular loop with an evacuation orifice for the synthesis gases and excess liquid phase, the diameter of which is less than the internal diameter of the said connection or tubular loop. This constriction at the point of evacuation of the gases creates a sort of flooding or obstruction which causes entraining of the synthesis gases such that the liquid phase in the descending branch is always saturated with gas. As an indication, it is estimated that the fraction, by volume, of gas entrained represents between 0.2 and 1% of the volume of liquid in the descending branch.